There is a long felt need for an economically viable process to convert acetic acid to acetaldehyde. Acetaldehyde is an important commodity feedstock for a variety of industrial products. For instance, acetaldehyde can readily be hydrogenated to ethanol, which in itself has wide variety of industrial applications including its wide utility as a gasoline additive. Acetaldehyde can also be converted to ethyl acetate by the Tischenko reaction or reacted with other compounds to form other products. Currently acetaldehyde is produced by the oxidation of ethylene, the Wacker oxidation of ethylene. Fluctuating natural gas and crude oil prices contribute to fluctuations in the cost of conventionally produced, petroleum or natural gas-sourced acetaldehyde, making the need for alternative sources of acetaldehyde all the greater when oil prices rise. Thus it is of interest to develop commercially viable routes to selectively hydrogenate acetic acid to acetaldehyde.
The catalytic hydrogenation of aromatic carboxylic acids to produce aromatic aldehydes has been reported in the literature. For instance, U.S. Pat. No. 4,613,700 to Maki et al. discloses that aromatic aldehydes can be formed from aromatic carboxylic acids using a catalyst comprising zirconium oxide containing as an essential component at least one element selected from the group consisting of chromium, manganese, iron, cobalt, zinc, bismuth, lead rhenium and the elements of Group III in periods 3 to 6 of the periodic table. However, no examples of catalytic hydrogenation of aliphatic carboxylic acids such as acetic acid are provided in this disclosure.
U.S. Pat. No. 5,306,845 to Yokohama et al. discloses a method of producing an aldehyde, which comprises hydrogenating a carboxylic acid or its alkyl ester with molecular hydrogen in the presence of a catalyst containing chromium oxide of high purity having a specific surface area of at least 10 m2/g and a total content of sodium, potassium, magnesium and calcium of not more than 0.4 weight percent. It is further reported therein that the hydrogenation reaction is conducted while maintaining the carboxylic acid or its alkyl ester at a concentration of not more than 10 volume percent. Additionally, the only example reported therein is hydrogenation of stearic acid to stearyl aldehyde. Most importantly, the selectivity to aldehyde drops significantly even if the total content of sodium, potassium, magnesium and calcium increases from about 0.3 weight percent to about 0.46 weight percent, thus rendering the process not suitable for a commercial operation.
U.S. Pat. No. 5,476,827 to Ferrero et al, describes a process for the preparation of aldehydes by catalytic hydrogenation of carboxylic acids, esters or anhydrides utilizing a bimetallic ruthenium/tin catalyst. The preferred carboxylic acids are the α-β-unsaturated carboxylic acids with an aromatic back bone or aromatic carboxylic acids. In Example 14, hydrogenation of acetic acid at 400° C. over a Ru—Sn/SiO2 catalyst comprising 0.6 wt. % Ru with a Sn/Ru mole ratio of 4.0 is reported to produce acetaldehyde with a selectivity for acetaldehyde of only 30%.
U.S. Pat. No. 6,121,498 to Tustin et al. discloses a method for producing acetaldehyde from acetic acid. In this process, acetic acid is hydrogenated with hydrogen at an elevated temperature in the presence of an iron oxide catalyst containing between 2.5 and 90 weight percent palladium. However, the optimal condition reported therein is comprised of an iron oxide catalyst containing at least about 20 weight percent palladium, which affords about 80 percent selectivity to acetaldehyde with about 50 percent conversion of acetic acid. Additionally, significant amounts of by-products including methane, ethane, ethylene, ethanol and acetone are formed.
From the foregoing it is apparent that existing processes do not have the requisite catalysts to selectively convert acetic acid to acetaldehyde or existing art employs catalysts, which are either expensive and/or non-selective for the formation of acetaldehyde and produces undesirable by-products.